I love Christina Aguilera's voice. I could listen to it forever and never get bored of it. She does oversing a lot, but I especially like her more gentle voice, which can be heard on her song 'Save Me From Myself" on her new album- no "yelling", no continuous trills, as Dan put it. She's flat out singing the whole time, almost whispering, and it's gorgeous.
I like a few of Britney's songs but that has more to do with the fact that she had the best producers in the business dolling her songs up, not that she is talented or can sing. She can dance, and she is cute (or was), but other than that she's got nothing.